Leukocytes entering a site of inflammation migrate between tightly apposed endothelial cells into the underlying tissues, a process called transendothelial migration (TEM). The inflammatory response is one of the bodies prime defenses against microorganisms, and is critical for normal wound repair. On the other hand, most human pathology involves inflammation that damages the host either intentionally (as in autoimmune phenomena) or accidentally, when the host is an innocent bystander. My long-term objectives are to understand, on a molecular level, what controls inflammation in general, and TEM in particular. To this end, I have been studying platelet/endothelial cell adhesion molecule-1 (PECAM-1/CD31), a 130 kD glycoprotein of the Immunoglobulin gene superfamily that is critical for TEM of most Mo, PMN, and NK cells. TEM involves homophilic interactions between domains 1 and/or 2 of leukocyte PECAM with the same domains of endothelial PECAM. Subsequent migration of leukocytes across the basal lamina involves heterophilic interaction of domain 6 of leukocyte PECAM with unknown ligand(s) in the matrix. The specific aims of this competitive renewal relate to how these molecular domains of PECAM carry out their function. 1. We have found that there is a continuous and active recycling of PECAM- bearing membrane in the junction. This membrane appears to be recruited for TEM events. We will study how this membrane trafficking is involved in TEM and the role that PECAM plays in it. 2. PECAM is arranged in molecular clusters along the endothelial junction. We will investigate whether and how this clustering is important for PECAM functions. 3. Since distinct domains of PECAM mediate different functions in leukocyte emigration, we will study whether domain-specific signals are transmitted by PECAM to regulate these processes. 4. We will continue to define the role of PECAM in vivo. We will test the therapeutic effect of blocking PECAM in two murine models of chronic inflammation-atherosclerosis and collagen-induced arthritis. These studies will shed light on how PECAM, a controller of inflammation, is itself regulated.